Summary of Work: Work begun during the previous reporting period on the use of three-dimensional reconstruction methods to improve spatial resolution in small-animal positron emission tomography (PET) studies yielded the highest resolution PET images ever made of a biological specimen (mouse skeleton, better than 1 mm). The components for a pixelated PET Buismuth Germinate Ortho (BGO) imaging system were obtained, the detectors and data acquisition system assembled, and preliminary system testing, calibration, and software development undertaken. This device, which is nearing completion, will permit small- animal imaging with a spatial resolution of about 2 mm in both tomographic and dynamic projection imaging modes. Pixelated arrays of both Lutetium Silicate Ortho (LSO) and BGO were coupled to miniature metal can position-sensitive photomultiplier tubes and evaluated in time coincidence. The scintillation properties of LSO were found to be extremely favorable and the pixel of interaction could be identified correctly more than 95% of the time at 511 keV. Data acquired during these benchtop studies have suggested several system designs that, in conjunction with novel data acquisition methods also under investigation, may lead to relatively inexpensive, very high- performance small-animal PET scanners. The elements for an imaging probe system and for an advanced slab LSO/position-sensitive PET imaging system were acquired, and development work on both devices was initiated.